How to judge index health

Hi
i just want to know how to judge index is healthy
i know i should look at
BLEVEL , LEAF_BLOCKS , DISTINCT_KEYS , CLUSTERING_FACTOR
in dba_indexes
but how to calculate from this value the health of an index
I know that BLEVEL should be 3 or less , any other rules?
Thanks very much

Thanks for all replies , BLEVEL is 4 or more means u need to rebuild ur index.
After reading , i am including example of more jnior dbas to help them understand , wen i can say that clustering factor is good or bad for an index
Ex:
suppose i have T1 table , and i have Id.T1 indexed with index called IDX1 ,
IF
select blocks from dba_tables where table_name ='T1';
= or close to
select clustering_factor from dba_indexes where index_name = 'IDX1';
then the clustering factor is very good. because that means each visit to 1 index block
will nearly fetch 1 data block from the table (data in the table is not randomly inserted accoring to the indexed column(ID))
but if the select clustering_factor from dba_indexes where index_name = 'IDX1';
is much higher than the blocks of the table and nearly = to the number of index entries (or the count(distinct values to the indxed column) (ID) )
then the clustering factor is very bad and the CBo may prefer to Full table scan rather than access the table by index

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Hi Suresh,
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Index Range Scan
select * from VVBAP here vbeln = '00123' ......end select.
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To execute the SQL statement, the DBMS first reads a root block (1) and a branch block (2). The branch block contains pointers to two leaf blocks (3 and 4). In order to find the index records that fulfill the criteria in the WHERE clause of the SQL statement, the DBMS searches through these leaf blocks sequentially. The index records found point to the table records within the table blocks (5 and 6).
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DB Indexex :Concatenation
In the concatenation access strategy, one index is reused. Therefore, various index search strings also exist. An index unique scan or an index range scan can be performed for the various index search strings. Duplicate entries in the results set are filtered out when the search results are concatenated.
Select * from vvbap where vbeln in ('00123', '00133', '00134').
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In the SQL statement above, a WHERE condition with an IN operation is specified over field VBELN. The fields MANDT and VBELN are shown on the left of the primary index. Various index search strings are created, and an index range scan is performed over the primary index for each index search string. Finally, the result is concatenated.
Full Table Scan
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For a full table scan, the read table blocks are added to the end of an LRU list. Therefore, no data blocks are forced out of the data buffer. As a result, in order to process a full table scan, comparatively little memory space is required within the data buffer.
The full table scan access strategy is very effective if a large part of a table (for example, 5% of all table records) needs to be read. In the example above, a full table scan is more efficient than access using the primary index.
In Brief
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Index range scan: The index selected is unique or non-unique. For a non-unique index, this means that not all index fields are specified in the WHERE clause. A range of the index is read and checked. An index range scan may not be as effective as a full table scan. The table records returned can range from none to all.
Full table scan: The whole table is read sequentially. Each table block is read once. Since no index is used, no index blocks are read. The table records returned can range from none to all.
Concatenation: An index is used more than once. Various areas of the index are read and checked. To ensure that the application receives each table record only once, the search results are concatenated to eliminate duplicate entries. The table records returned can range from none to all.
Regards,
Balaji Reddy G
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| Id | Operation | Name | Rows | Bytes | Cost |
| 0 | SELECT STATEMENT | | 1 | 8 | 194 |
| 1 | SORT AGGREGATE | | 1 | 8 | |
| 2 | TABLE ACCESS FULL | EMP | 91000 | 710K| 194 |
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| 0 | SELECT STATEMENT | | 1 | 8 | 39 |
| 1 | SORT AGGREGATE | | 1 | 8 | |
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Regards

Dear Akshay,
Below is the information about indexes and there creation for ODS.
You can search a table for data records that satisfy certain search criteria faster using an index.
An index can be considered a copy of a database table that has been reduced to certain fields. This copy is always in sorted form. Sorting provides faster access to the data records of the table, for example using a binary search. The index also contains a pointer to the corresponding record of the actual table so that the fields not contained in the index can also be read.
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Leaving content frame.
With Regards,
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SYS_C0016575                   NORMAL                      UNIQUE
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2 rows selected.

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